Optically variable image devices currently include holograms, kinegrams, pixelgrams and interference gratings (e.g., variable and minutely spaced parallel lines). Because holograms are the better known of such devices, the description herein will refer primarily to holograms and holographic images. However, it is to be understood that the present invention is not limited in its application to holograms, but is broadly applicable to all such optically variable image devices.
Since Dennis Gabor of Great Britain introduced holography in 1948, and Yuri N. Denisyuk of Russia produced reflective holograms in 1962, holograms and other optically variable image devices have found applications in a variety of areas. In response to security application demand, intensive research and development have been devoted to commercial mass production of reflective holograms by embossing or casting and curing a relief pattern onto a transparent polymer substrate, such as polyester, polyvinyl chloride and acrylate polymers, to provide the interference pattern required for a holographic image. The most recent developments are represented by U.S. Pat. Nos. 5,085,514, 4,973,113, 4,933,120 and 4,913,858. By illuminating the interference pattern on the substrate, the film will reconstruct and display the holographic image at one or more angles of observation.
One important security application is to laminate semitransparent reflective holograms onto security documents, such as credit cards and identification badges, so that the document can be read through the hologram and the holographic image provides security advice for the document. Due to the optically variable characteristic of the holographic image, the document cannot be photocopied or counterfeited since the holographic image cannot be photographically reproduced. Likewise, the document cannot be altered because any attempt to remove the hologram bearing layer will result in destruction of the holographic image. Thus the protected document enjoys a high level of security.
In order to protect the image-carrying relief pattern on the substrate and to laminate the hologram onto documents, the image bearing surface has to be coated or laminated by means of a transparent substance. Unfortunately, direct coating or laminating of a transparent polymer with an adhesive will almost totally erase an unprotected or nonenhanced holographic image due to the fact that most visually transparent polymers and resins have an optical constant in the range of 1.45 to 1.65.
Optical science indicates that this problem can be overcome by applying a substantially transparent dielectric or semiconductor layer with high reflective index, and appropriate thickness, on the holographic relief pattern to obtain the visual holographic image in reflection. Optical science also indicates that a thin metal layer can be semitransparent and still enhance the reflection, but with lower quality in comparison with dielectric and semiconductor layers. For a given level of reflectivity, metals are too absorptive for transparent or semitransparent applications. However, metal layers are useful for high absorbency low transmission applications. See for example U.S. Pat. Nos. 4,315,665 and 4,840,444 which utilize highly absorbing metal coatings on holographic images for use in low transmittance applications, such as sunglasses and solar control window films.
U.S. Pat. No. 4,856,857 is directed to a transparent hologram comprising a transparent substrate bearing a holographic interference relief pattern, and a thin image enhancing film applied to and following the form of the relief pattern and having an index of refraction different from that of the substrate by more than 0.2, and preferably about 1.0 or more. The thin image enhancing film may be applied by vapor deposition and in other manners, and is said to be applied to a thickness not exceeding 200 angstroms. The thin film may be selected from a broad range of materials listed in the patent, including inter alia, aluminum, silver, copper, titanium oxide (TiO.sub.2), zinc oxide (ZnO), and bismuth oxide (Bi.sub.2 O.sub.3).
As above noted, the use of reflective metals such as aluminum, even when applied in extremely thin layers, has not met with wide spread acceptance for transparent holographic applications because of the absorbency of the metal. Zinc oxide and titanium oxide, and niobium oxide as well, have proven useful as image enhancers. However, zinc oxide, especially when laminated to a base card, cuts down on image visibility and sharpness and produces a product of low quality. Niobium oxide and titanium oxide produce a better product, but the production rate is slow and excessively costly. Little use appears to have been made of Bi.sub.2 O.sub.3, because of low transmittance and high absorbency at wavelengths less than 600 nanometers.
In commercial practice today, a more successful alternative appears to be the process represented by U.S. Pat. No. 5,044,707, which utilizes a metallization/demetallization technique. The process involves an environmentally unfriendly chemical etching process, and results in a limited yield. The image produced is grayish, which is not favorable. Another alternative appears in U.S. Pat. No. 5,087,510, which proposes an electrolessly deposited thin metal film for mass production of image enhanced reflective holograms. Again, the process involves environmentally unfriendly aspects and results in limited yield. Moreover, the absorption of the metal layer makes the production of a top quality, clear, reflective hologram inherently improbable.